The present invention is directed to an apparatus and method for enhancing a video signal and, more specifically, to an apparatus and method for improving the sharpness of a video image by making edge transitions steeper.
The television industry is undergoing significant changes as a result of the transition from the current standard definition television (SDTV) to high definition television (HDTV). Much of this change is driven by the FCC requirement that all broadcasters in the United States must transmit all programming content as HDTV signals and must cease transmitting SDTV signals by the year 2006.
As a result, high definition televisions are becoming increasingly available in the marketplace, as are HDTV conversion systems that convert an HDTV signal to an SDTV image for display on a standard definition television.
Some of the driving forces behind the transition to HDTV are the possibility of a larger and clearer picture, the changed aspect ratio (similar to movie format) in some systems, and the decreased susceptibility of the digital signal to noise during transmission to the viewer. As screens grow larger, viewers expect increased resolution. However, for a number of years to come, HDTV sets must be able to receive and display television signals according to the existing SDTV standard (e.g., PAL, NTSC, SECAM) while broadcast facilities are making the transition to the new HDTV standard (ATSC). In the interim, it is highly desirable that an HDTV set be able to display an SDTV signal at increased resolution to create the subjective impression of a high definition television image.
In addition, from the broadcast side, techniques are needed which can up-convert existing standard definition (SD) materials into high definition (HD) format.
Unfortunately, the resolution of the video signal at the television receiver is limited by the quality of the original video signal (e.g., PAL, NTSC, SECAM) or the bandwidth of the transmission channel. Therefore, in order to increase the resolution of the SDTV signals for better perceptual quality, post processing the video signal in the receiver after demodulation becomes increasingly important.
Post processing techniques for improving the resolution of a video image involve expanding the original signal bandwidth to match HD signal format. One approach is to sharpen the edges or boundaries of objects in the video image, such as people, plants, buildings, furnishings, and the like. In order to sharpen edges, a number of edge detection techniques have been developed. These techniques may be grouped in four principal categories: gradient operators, compass operators, Laplace operators and stochastic gradient operators. The Laplace operator locates edges the most precisely. However, because of its sensitivity to noise, the Laplace operator sometimes detects too many false edges.
Additionally, some objects in a video image, such as background objects, are deliberately kept out of focus. Other objects, such as peoples faces, have smooth transients. The edges of these objects are called soft edges or smooth edges. To keep the naturalness of these objects the same as in the original picture, it is important that the edge enhancement techniques be capable of adapting to the local edge features.
Luminance transient improvement (LTI) is a conventional technique for sharpening a video image by steepening edge transitions, which thereby increases the original signal bandwidth. There are two general approaches to making an edge steeper. One technique increases pixel values on the high side of any edge and decreases pixel values on the other side of the edge, thereby making a gradual transition more abrupt. The other technique is to replace pixel values near the edge with pixel values from further away from the edge. In steepening an edge, the LTI algorithm creates additional high frequency components around the edges.
However, there are two major drawbacks for existing algorithms. First, due to discrete time sampling, the edge center does not necessarily fall on the pixel lattice. If uncertainty in the exact location of the edge becomes too large, jitter becomes visible after enhancement. Secondly, the over enhancement of soft edges may result in an unnatural looking image.
There is therefore a need in the art for systems and methods for improving the resolution of a video image. In particular, there is a need for improved systems and methods for sharpening edges in a video image without introducing additional artifacts. More particularly, the improved system should be able to locate an edge center at the subpixel level and should be able to adapt to the local edge features.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide an apparatus for sharpening an edge in a video image. In an advantageous embodiment of the present invention, the apparatus comprises: 1) a first circuit capable of determining a first luminance value of a first pixel associated with the first edge and a second luminance value of a second pixel associated with the first edge, wherein the first and second pixels are adjacent pixels; 2) a second circuit capable of determining a position of a first subpixel disposed between the first and second pixels, wherein the first subpixel position is disposed approximately at a center of the first edge; 3) a third circuit capable of increasing a luminance value of a second subpixel disposed on a first side of the first edge center; and 4) a fourth circuit capable of decreasing a luminance value of a third subpixel disposed on a second side of the first edge center opposite the first side.
According to one embodiment of the present invention, the second circuit determines the position of the first subpixel by calculating a first second-order derivative of luminance for the first pixel and the second pixel.
According to another embodiment of the present invention, the second circuit determines the position of the first subpixel by calculating a plurality of intermediate second-order derivatives of luminance at a plurality of subpixel positions disposed between the first and second pixels.
According to still another embodiment of the present invention, the second circuit calculates the plurality of intermediate second-order derivatives of luminance at the plurality of subpixel positions by using a linear interpolation of the first second-order derivative.
According to yet another embodiment of the present invention, the second circuit determines the position of the first subpixel by determining a selected one of the subpixel positions at which a corresponding one of the plurality of intermediate second-order derivatives of luminance has a minimum value.
According to a further embodiment of the present invention, the second circuit calculates the plurality of intermediate second-order derivatives of luminance at the plurality of subpixel positions by using a polynomial interpolation of the first second-order derivative of luminance.
According to a still further embodiment of the present invention, the apparatus further comprises a gain control circuit capable of adjusting a value of luminance associated with at least one of the second subpixel and the third subpixel according to a frequency of additional edges proximate the first edge.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.